Start-Up Supply

ABSTRACT

In one embodiment, an apparatus includes a transistor having a gate, a drain, and a source. The drain is coupled to receive an AC power supply signal. A component is coupled between an output node and the gate of the transistor. The component couples an output voltage from the output node to charge a gate-source capacitor during a first portion of the AC power supply signal. The transistor is configured to turn on during a second portion of the AC power supply signal to send a charge to the output node where the charge is used to power a circuit of a power supply.

CROSS REFERENCE TO RELATED APPLICATIONS

The present disclosure claims priority to U.S. Provisional App. No.61/289,897 for “High-Input Voltage SMPS Start-Up Supply” filed Dec. 23,2009, which is incorporated herein by reference in its entirety for allpurposes.

BACKGROUND

Particular embodiments generally relate to power supplies.

Unless otherwise indicated herein, the approaches described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

A switched mode power supply (SMPS) regulates an input voltage toprovide an output voltage. A power factor correction (PFC) controller isused in the conversion and regulation. The PFC controller typicallyrequires a minimumsupply voltage to operate. During operation, theoutput voltage of the SMPS is used to supply the required operatingvoltage for the PFC controller. However, at certain times, such asduring the SMPS power-up or when a device is in a standby mode (e.g.,the output voltage of the SMPS is down), an input voltage is needed fora PFC controller.

A start-up supply may be used to supply the operating voltage for thePFC controller during the start-up and when the device is in the standbymode. The operating voltage is supplied until the SMPS powers up. Afterthe SMPS powers up, the start-up supply is then deactivated until neededagain.

SUMMARY

In one embodiment, an apparatus includes a transistor having a gate, adrain, and a source. The drain is coupled to receive an alternatingcurrent (AC) power supply signal. A component is coupled between anoutput node and the gate of the transistor. The component couples anoutput voltage from the output node to charge a gate-source capacitorduring a first portion of the AC power supply signal. The transistor isconfigured to turn on during a second portion of the AC supply signal tosend a charge to the output node where the charge is used to power acircuit of a power supply.

In one embodiment, the component includes a first component and theapparatus further includes a second component coupled to the source ofthe transistor and the output node. The second component causes thesource of the transistor to follow the AC power supply signal until thetransistor turns on.

In one embodiment, a switch is configured to be controlled to dischargethe gate-source capacitor.

In one embodiment, a system includes a capacitor configured to becharged when the transistor is turned on.

In one embodiment, a method includes: coupling an AC power supply signalto a transistor; coupling an output voltage from an output node to agate of the transistor to charge a gate-source capacitor; and turningthe transistor on to send a charge to the output node, the charge beingused to power a circuit of a power supply.

The following detailed description and accompanying drawings provide amore detailed understanding of the nature and advantages of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example of a switched mode power supply (SMPS)according to one embodiment.

FIG. 2 depicts a more detailed example of the start-up supply accordingto one embodiment.

FIG. 3 depicts waveforms of the start-up supply according to oneembodiment.

FIG. 4 depicts an example of a start-up supply to provide a path forcharging gate-source capacitor Cgs from the AC power supply according toone embodiment.

FIG. 5 depicts a simplified flow chart of a method for operatingstart-up supply according to one embodiment.

FIG. 6 depicts a simplified flowchart of a method for providing start-upassistance according to one embodiment.

DETAILED DESCRIPTION

Described herein are techniques for a start-up supply. In the followingdescription, for purposes of explanation, numerous examples and specificdetails are set forth in order to provide a thorough understanding ofembodiments of the present invention. Particular embodiments as definedby the claims may include some or all of the features in these examplesalone or in combination with other features described below, and mayfurther include modifications and equivalents of the features andconcepts described herein.

FIG. 1 depicts an example of a switched mode power supply (SMPS) 100according to one embodiment. A start-up supply 102, a power factorcorrection controller 104, and an alternating current (AC) power supplycircuit 106 are provided. Although a switched mode power supply isdiscussed, particular embodiments may be used with other power supplyconfigurations that require start-up supply 102.

AC power supply circuit 106 provides an AC signal to a node VAC ofstart-up supply 102. As will be described in more detail below, ahalf-rectified signal may be provided to node VAC.

Start-up supply 102 is configured to receive the AC power supply signaland provide an output signal at a node VOUT to charge a capacitor CL.Capacitor CL is charged to supply a sufficient voltage to PFC controller104 during times when SMPS 100 is not supplying a sufficient auxiliaryvoltage. The auxiliary voltage may be from the output voltage of SMPS100, which is different than the output voltage Vout of start-up supply102. The auxiliary voltage of

SMPS 100 may be the voltage that is being supplied to a computing devicebeing powered by SMPS 100. The output voltage from start-up supply 102and the auxiliary voltage may be a direct current (DC) voltage.

Start-up supply 102 may supply the necessary charge to capacitor CL,which provides the charge to PFC controller 104. The charge may besupplied during power-up of the computing device and also when thecomputing device is in a standby mode. When the computing device isstarting up or in standby mode, the auxiliary voltage is down.

When SMPS 100 has started up and is providing a sufficient auxiliaryvoltage, the auxiliary voltage can be used to provide power to PFCcontroller 104. At this point, start-up supply 102 may not be needed.Thus, start-up supply 102 may be turned off (e.g., a transistor (notshown) in start-up supply 102 is turned off) such that start-up supply102 is not supplying charge to capacitor CL. This state continues untilstart-up supply 102 is needed again, such as when the computing deviceis powered down and restarted, or when the computing device is in thestandby mode and is restarted.

Start-up supply 102 may be included on an integrated circuit (IC) chipthat includes three pins, a pin for node VAC, a pin for node VOUT, and apin for ground (GND). By using only three pins, pin count is limited forthe chip. Also, as will be described in more detail below, a voltagefrom node VOUT is used to provide a necessary voltage to charge agate-source capacitor of a transistor (not shown) in start-up supply102.

FIG. 2 depicts a more detailed example of the start-up supply 102according to one embodiment. AC power supply circuit 106 includes an ACpower source 202 and a bridge rectifier 204. AC power source 202 mayprovide an AC signal, such as a 220 volt root mean square (RMS) signal.Bridge rectifier 204 includes diodes D1, D2, D3, and D4. Bridgerectifier 204 may be a half wave rectifier, which takes the AC supplysignal and blocks the negative half of the AC supply signal. In thiscase, the positive half of the AC supply signal is provided to node VACof start-up supply 102.

A transistor Q1 receives the rectified AC power supply signal from nodeVAC and provides an output voltage at node VOUT. Transistor Q1 has itsdrain coupled to node VAC. Also, the body of transistor Q1 is coupled toa source of transistor Q1 and a diode D7 is coupled through the body tothe drain of transistor Q1. A gate-source capacitor Cgs is shown torepresent the capacitance between the gate and source of transistor Q1.

A diode D5 couples the source to the output node VOUT. Also, the outputnode VOUT is coupled to the gate of transistor Q1 through a diode D6.Particular embodiments use the output voltage VOUT to charge gate-sourcecapacitor Cgs to a necessary drive voltage during at least a portion ofthe AC power supply signal. For example, as will be discussed in moredetail below, the gate-source capacitor Cgs is charged while the ACpower supply signal is negative. This allows transistor Q1 to turn onduring a phase when conduction is permitted, but before the AC voltagebecomes sufficient enough to start charging capacitor CL. The turn ontime is determined by a conduction angle, which is the portion of acycle of the AC power supply signal during which the transistor Q1conducts.

The operation of start-up supply 102 will be described with respect toFIG. 2 and FIG. 3. FIG. 3 depicts waveforms of SMPS 100 according to oneembodiment. A graph 302 shows the AC power supply signal, a graph 304shows the rectified AC power supply signal, a graph 306 shows thevoltage at the source of transistor Q1, and a graph 308 shows thevoltage at the gate of transistor Q1.

During the negative half cycle of the AC supply signal, the drain oftransistor Q1 is held at a potential below ground (e.g., by a diode dropvia diode D7) for a part or the entire period based on the load oncapacitor CB on bridge rectifier 204.

When the voltage at node VAC is zero, gate-source capacitor Cgs ischarged to the output voltage through diode D6. For example, the outputvoltage turns diode D6 on and capacitor Cgs is charged. A charging pointis shown at 310 where the AC supply signal is negative.

When the positive cycle of AC supply signal starts, the VAC voltage atnode VAC increases above zero. The source of Q1 (node VS) also followsthe VAC voltage at node VAC due to having diode D5 in place. Forexample, diode D5 may be reverse biased until the source of transistorQ1 is sufficient to forward bias diode D5. The source of transistor Q1follows the VAC voltage until the voltage VS becomes equal to a voltageVOUT (the diode drop across diode D5 is ignored for discussion purposes)when transistor Q1 starts conducting to charge capacitor CL. The voltageacross gate-source capacitor Cgs does not change when the source oftransistor Q1 moves above zero because there is no path from the gate oftransistor Q1 to discharge gate-source capacitor Cgs. Accordingly, thevoltage at the gate of transistor Q1 continues to provide sufficientdrive to have transistor Q1 conducting.

Transistor Q1 conducts for a certain phase (according to the conductionangle) of the AC power supply signal. When the conduction angle ends,transistor Q1 is turned off to stop charging capacitor CL. Theconduction angle is used to increase efficiency. For example, theefficiency is greater when the VAC voltage is smaller. Accordingly, asshown in FIG. 3, capacitor Cgs is discharged and transistor Q1 is turnedoff when the conduction angle is reached. At this point, capacitor CL isnot being charged by start-up supply 102. Switch 51 may be closed toprovide a path to discharge gate-source capacitor Cgs. The discharge ofgate-source capacitor Cgs will be described in more detail below. Byhaving transistor Q1 be OFF for the remaining portion of the AC powersupply signal cycle after charging the output voltage VOUT adequately,higher power efficiency is achieved for the charging process.

The above process continues when the AC power supply signal goesnegative and gate-source capacitor Cgs is charged. Then, transistor Q1is turned on to charge capacitor CL when voltage VS becomes equal to avoltage VOUT.

As discussed above, the charge across capacitor CL is used to supply avoltage to PFC controller 104. The above process continues until SMPS100 is powered up and a sufficient auxiliary voltage being output bySMPS 100 can be supplied to PFC controller 104. The auxiliary voltagemay then be used to charge capacitor CL. At this point, transistor Q1 isturned off until it is needed again to provide a start-up charge.Another important part is that the

Referring back to FIG. 2, output regulation and conduction angleregulation will be described in more detail. Output voltage regulationmay be provided by a resistor R1, a resistor R2, and a comparator COMP1.Also, conduction angle regulation may be provided by a resistor R3, aresistor R4, and a comparator COMP2.

Output regulation is used to determine when to control switch S1 to turnoff transistor Q1. At this point, SMPS 100 may be able to provide theauxiliary voltage to power PFC controller 104. In one embodiment, when avoltage input into the positive terminal of comparator COMP1 reaches acertain level as compared to a voltage reference VREF, switch S1 iscontrolled to be closed. In this case, gate-source capacitor Cgs cannotbe charged to allow transistor Q1 to conduct. For example, when startingup, voltage VOUT may be below a voltage that causes the input intocomparator COMP1 to be below the voltage reference VREF (via resistordivider of resistors R1 and R2). When SMPS 100 can supply the auxiliaryvoltage, voltage VOUT goes above a level where the input into comparatorCOMP1 goes above the voltage reference VREF. Comparator COMP1 thenoutputs a logic high signal. Logic gate (e.g., Or gate) 206 outputs alogic high signal to a level shifter 208. Level shifter 208 may be usedto shift the voltage level to a level that may turn on a transistor (notshown) acting as the switch to close switch S1.

In conduction angle regulation, when a voltage input into the positiveterminal of comparator COMP2 reaches a certain level compared withvoltage reference VREF, switch S1 is controlled to discharge capacitorCgs. For example, when the VAC voltage reaches a certain level, switchS1 is closed to discharge gate-source capacitor Cgs according to theconduction angle. The VAC voltage is divided by a resistor dividernetwork of resistor R3 and resistor R4. When the input signal from theresistor divider network into comparator COMP2 goes above referencevoltage VREF, comparator COMP2 outputs a logic high signal. Logic gate206 outputs a logic high signal to level shifter 208. In one example,the output from comparator COMP2 may be a logic low level at this point(e.g., because the voltage VOUT is lower then reference voltage VREFbecause voltage VOUT has not reached the desired level during start up).Level shifter 208 shifts the voltage level to a level that may turn onthe transistor (not shown) to close switch S1.

When the VAC voltage goes below a certain level, the signal input intocomparator COMP1 goes below reference voltage VREF. Comparator COMP2then outputs a logic low level, which turns off the transistor (notshown) and opens switch S1. The above process continues as switch S1 isclosed and opened according to the conduction angle.

At certain times, such as before power-up of SMPS 100, capacitor CL isrelaxed or does not include a charge across it. Thus, the output voltageVOUT cannot be used to charge gate-source capacitor Cgs. Accordingly,start-up assistance is used to charge gate-source capacitor Cgs. FIG. 4depicts an example of start-up supply 102 to provide a path for charginggate-source capacitor Cgs according to one embodiment. The VAC voltageis used to charge gate-source capacitor Cgs. Resistor R3 of FIG. 2 maybe modified into resistors R3 a, R3 b, and R3 c. This provides a pathfor charging gate-source capacitor Cgs from node VAC. For example, apath is provided through a resistor R3 c and a diode D8 to chargegate-source capacitor Cgs.

Once capacitor CL is charged fully for the first time, this path is notneeded. Rather, as was described above, output voltage VOUT is used tocharge gate-source capacitor Cgs. Thus, a switch S2 is used to de-couplethe path to charge gate-source capacitor Cgs. For example, when avoltage VOUT reaches a certain level, the output of comparator COMP1goes high and a latch 402 is used to control switch S2. For example,switch S2 is closed to couple resistor R3 c to ground. At this point,resistors R3 a, R3 b, R3 c, and R4 together determine the conductionangle. Also, diode D8 prevents charge from flowing from the outputvoltage through resistor R3 c.

FIG. 5 depicts a simplified flow chart 500 of a method for operatingstart-up supply 102 according to one embodiment. At 502, the outputvoltage from node VOUT is coupled to the gate of transistor Q1 to chargegate-source capacitor Cgs. At 504, transistor Q1 is turned on to chargecapacitor CL. At 506, when the end of the conduction angle is reached,gate-source capacitor Cgs is discharged.

FIG. 6 depicts a simplified flowchart 600 of a method for providingstart-up assistance according to one embodiment. At 602, the VAC voltageis coupled to the gate of transistor Q1. At 604, gate-source capacitorCgs is charged by the VAC voltage. At 606, switch S2 is closed whencapacitor CL is charged fully for the first time.

Accordingly, particular embodiments provide a start-up supply thatavoids a higher power dissipation because a second rectified signal thatis off-chip is not used to charge gate-source capacitor Cgs. Rather, theoutput voltage VOUT is used to charge gate-source capacitor Cgs. Thisavoids the use of an extra resistor that is coupled between the gate oftransistor Q1 and a pin that would be needed to couple the secondrectified signal to charge gate-source capacitor Cgs. Also, this lowersthe power dissipation and the pin count of the IC chip. Further, diodeD5 is placed on-chip which reduces the bill of materials (BOM) that isneeded to produce SMPS 100.

As used in the description herein and throughout the claims that follow,“a”, “an”, and “the” includes plural references unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise.

The above description illustrates various embodiments of the presentinvention along with examples of how aspects of the present inventionmay be implemented. The above examples and embodiments should not bedeemed to be the only embodiments, and are presented to illustrate theflexibility and advantages of the present invention as defined by thefollowing claims. Based on the above disclosure and the followingclaims, other arrangements, embodiments, implementations and equivalentsmay be employed without departing from the scope of the invention asdefined by the claims.

1. An apparatus comprising: a transistor having a gate, a drain, and asource, wherein the drain is coupled to receive an alternating current(AC) power supply signal; and a component coupled between an output nodeand the gate of the transistor, wherein the component couples an outputvoltage from the output node to charge a gate-source capacitor during afirst portion of the AC power supply signal, wherein the transistor isconfigured to turn on during a second portion of the AC power supplysignal to send a charge to the output node, the charge used to power acircuit of a power supply.
 2. The apparatus of claim 1, wherein thecomponent comprises a first component, the apparatus further comprisinga second component coupled to the source of the transistor and theoutput node, wherein the second component causes the source of thetransistor to follow the AC power supply signal until the transistorturns on.
 3. The apparatus of claim 2, wherein the first componentcomprises a first diode and the second component comprises a seconddiode.
 4. The apparatus of claim 1, wherein the transistor is configuredto turn off for a third portion of the AC power supply signal.
 5. Theapparatus of claim 4, wherein: the second portion comprises a first timeperiod when the gate-source capacitor is charged; and the third portioncomprises a second time period when the gate-source capacitor isdischarged.
 6. The apparatus of claim 1, further comprising a switchconfigured to be controlled to discharge the gate-source capacitor. 7.The apparatus of claim 6, further comprising a conduction angle circuitconfigured to control the switch to discharge the gate-source capacitorwhen the AC power supply signal reaches a threshold.
 8. The apparatus ofclaim 6, further comprising an output regulation circuit configured tocontrol the switch to keep the gate-source capacitor discharged when theoutput voltage reaches a threshold.
 9. The apparatus of claim 1, whereinthe circuit comprises a first circuit, the apparatus further comprisinga second circuit configured to charge the gate-source capacitor, thesecond circuit coupling the AC power supply signal to the gate of thetransistor.
 10. The apparatus of claim 9, wherein the second circuitcouples the AC power supply signal to the gate of the transistor when anexternal capacitor coupled to the output node does not have a chargeacross the external capacitor until the output voltage reaches athreshold.
 11. The apparatus of claim 9, wherein the second circuitcomprises: a resistor coupled to the AC power supply signal; and asecond switch; and a second switch control circuit configured to controlthe second switch to couple the resistor to ground.
 12. The apparatus ofclaim 1, wherein the apparatus comprises three pins, wherein the threepins comprise: a first pin coupled to the AC power supply signal; asecond pin coupled to the output node; and a third pin coupled toground.
 13. A system comprising the apparatus of claim 1, the systemcomprising: a capacitor configured to be charged when the transistor isturned on.
 14. The system of claim 13, wherein: the circuit comprises apower factor correction circuit configured to use an auxiliary outputvoltage from the power supply to provide a charge to the capacitor, andthe transistor is configured to not provide the charge to the outputnode when the auxiliary output voltage is at a level to provide thecharge to the capacitor.
 15. A method comprising: coupling an AC powersupply signal to a transistor; coupling an output voltage from an outputnode to a gate of the transistor to charge a gate-source capacitor; andturning the transistor on to send a charge to the output node, thecharge used to power a circuit of a power supply.
 16. The method ofclaim 15, further comprising discharging the gate-source capacitor whenthe AC power supply signal reaches a threshold.
 17. The method of claim15, turning off the transistor when the output voltage reaches athreshold.
 18. The method of claim 15, further comprising coupling theAC power supply signal to the gate of the transistor to charge thegate-source capacitor.
 19. The method of claim 18, wherein the AC powersupply signal is coupled to the gate of the transistor when an externalcapacitor coupled to the output node does not have a charge across theexternal capacitor until the output voltage reaches a threshold.
 20. Themethod of claim 15, wherein: the circuit comprises a power factorcorrection circuit, and the charge is provided to the output node tocharge a capacitor, the capacitor configured to provide a charge to thepower factor correction circuit.